Scientific consensus holds that liquid water cannot exist on the surface of Mars. But now a pair of scientists argue that liquid water in limited amounts and for limited times may indeed be present on Mars' surface.

DNA is the building block for life on Earth. But it is a highly complex molecule, and could not have arranged itself spontaneously. What did it develop from? Astrobiologists examine possible ancestors of DNA: nucleic acids called PNA, p-RNA, and TNA.